This invention relates to a method of recovering tantalum of high purity from silicon-containing tantalum scrap.
In the production of single crystalline lithium tantalate which is useful for the material to constitute surface acoustic wave filter element, there has been increasing demand for tantalum pentoxide of high purity for the purpose of enhancing the yield of the same. It has been accordingly practiced to recover the tantalum from tantalum-containing scrap such as melts in crucible which are formed at the time when single crystalline lithium tantalate is produced, wafer cuttings produced at the time of the processing of single crystalline lithium tantalate, scrap from wafer grinding, and scrap from tantalum capacitor.
Tantalum-containing scrap generally includes as impurities silicon, iron, copper, aluminium, niobium, calcium and so on. As the method for recovering tantalum from such tantalum-containing scrap, the following extraction method and distillation method are known in general:
In the extraction method, tantalum-containing scrap is first dissolved, for example, in hydrofluoric acid, and then tantalum is extracted with methyl isobutyl ketone (MIBK).
Niobium which is extracted simultaneously therewith is removed by the back-extraction process by the use of a solution of sulfuric acid containing hydrofluoric acid. Thereafter, added is, for example, potassium salt such as potassium carbonate to precipitate tantalum as potassium fluorotantalate (K.sub.2 TaF.sub.7). The precipitate is separated by filtration, dissolved again in hot water of about 80.degree. C., and then hydrolized by adding aqueous ammonia thereto to obtain a precipitate of tantalum hydroxide (Ta(OH).sub.5). The thus obtained precipitate is collected by filtration and heated at higher temperature to obtain tantalum pentoxide (Ta.sub.2 O.sub.5). (See Carlson, C. W. & U. R. H. Nielsen, "J. Metals", June 1960, P. 472).
This extraction method, however, is disadvantageous in that niobium can not completely be removed at the time of the back-extraction using the solution of sulfuric acid containing hydrofluoric acid; the residual niobium tends to coprecipitate as potassium fluoroniobate when the potassium salt is added; and that the purity of tantalum pentoxide thus obtained as well as the yield thereof is lowered because of the solubility of potassium fluorotantalate in the solution of sulfuric acid containing hydrofluoric acid.
The distillation method, on the other hand utilizes the difference in boiling points of chlorides obtainable when tantalum-containing scrap is mixed, for example, with carbon, followed by chlorination at about 600.degree. to 800.degree. C. by passing a chlorine gas. (See German Patent Nos. 1017601 and 1056105.) In this method, however, it is difficult to control the temperature at the time of fractional distillation of the chlorides because the difference of boiling points between tantalum chloride (b.p. 239.degree. C.) and niobium chloride (b.p. 249.degree. C.), which is 10.degree. C., is too small from industrial viewpoints. In addition, it becomes substantially impossible to separate them if something having boiling point similar to the tantalum chloride is present, and therefore tantalum pentoxide of high purity may not be recovered.
Hague et al, "Journal of Research of the National Bureau of Standards", Vol. 62, No. 1, 1959, pp 11-19, teaches a method of separating tantalum metals by dissolving the sample in acid, contacting tantalum and other metals with an anion exchanger, eluting with a solution containing hydrofluoric acid and hydrochloric acid, and finally eluting tantalum with a solution containing ammonium salt. This publication, however, does not mention a recovery of tantalum from a scrap where the silicon, which otherwise tends to remain unremoved and contained in a tantalum eluate as impurity, is included.